Preliminary data show that sex hormone binding globulin (SHBG) concentrations in serum fall with age in the female rhesus monkey and reach a nadir in the sexually mature animal. There is no difference in SHBG concentrations in the follicular and luteal phases of the menstrual cycle. The metabolic clearance rate (MCR) of estradiol (assessed by the constant infusion technique) appears to be the same in Nembutal-anesthetized adult and immature female monkeys when expressed in liters/day/kg body weight. Further work must be done to determine the role of the site of blood sampling and the role of anesthesia in the measurement of the MCR E2.